Re: [PATCH 4/5] libav: upgrade to 9.18

[Phil Blundell <pb@pbcl.net>]

On Fri, 2015-05-29 at 16:28 +0100, Paul Eggleton wrote:
> Just for background, the reason I disabled the textrel check for x86 in 
> libav.inc was that I was able to determine based on quick research that 
> upstream deliberately doesn't enable -fPIC for x86 (32-bit) because apparently 
> it doesn't really work there. I honestly didn't check what the situation was 
> on 32-bit ARM; I probably should have done that at the time.

PIC is particularly problematic on x86 because it ties up %ebx.  This
represents 25% of the available general purpose registers which can
cause a measurable performance loss because of increased spilling, and
in the particular case of libav there is a lot of hand-written assembly
in the codecs which expects to be able to clobber that register.  It
certainly used to be the case that this code wouldn't compile at all
under -fpic.

The performance thing does apply to ARM as well but to a slightly lesser
extent.  The second problem probably doesn't affect ARM at all.  But,
personally, I would disable the textrel check and build without -fpic on
all platforms.

p.